A variety of structural parts and complete articles is conventionally made by a process wherein the shell of the article is first blow-molded or similarly cast or molded. Sometimes the shell is made in two or more complementary sections which are then seamed together about a perimetrical rim e.g. by thermal bonding, acoustic welding or by an adhesive, in order to define an internal cavity. Such a process is conventionally used, even though it is obviously cumbersome, because it gives the manufacturer a chance to flame-etch on the sections the surfaces that will become the internal surfaces of the shell cavity, for increasing the adhesion of the core foam thereto. Then, after this shell is fabricated, set or cooled and removed from the mold or fixture, a foamed plastic composition is injected through an opening purposely made or left filling the cavity with foam which sets. Finally, the opening is plugged completing the creation of an article which has a typically dense, tough, cleanable skin with a lightweight core which has the bonus of being thermally insulative. Typical uses for such articles are as bodies and lids of picnic chests, coolers, hospital trays, enclosures for hot or cold casseroles of food, containers for chemical, medical, biological and pharmaceutical specimens and products, refrigerator cabinet boxes and doors, food service containers and enclosures for food service containers, and the like.
A typical thermoplastic material used to make the shell of such articles is polyethylene, polypropylene or the like.
A typical foamed plastic core is made of polyurethane by using a commercially available Gusmer Gun in accordance with its manufacturer's instructions. (Gusmer Corporation, Route 18 and Spring Valley Road, Old Bridge, N.J. 08857). In a standard Gusmer Gun system, as shown in FIG. 1 of the drawings, pressurized air is supplied at 10 through a regulator 12 and valve 14 to an air motor 16 the piston rod 18 of which is yoked at 20 to the piston rods 22 of the piston-type proportionating pumps 24, 26 respectively for resin and activator. The pumps 24, 26 in turn have their suction sides respectively communicated to supply drums 28, 30 of resin and activator. The pressure sides of pumps 24, 26 are communicated through respective heaters 32, 34 to respective inlet ports of a mixing and ejecting gun 36. Within the gun 36 the properly proportioned streams of heated resin and activator come together in a mixing chamber 38 from the front of which the foamable mixture is ejected through a nozzle (not seen in this rear view). Typically in the Gusmer Gun system, the resin tank contains a prepolymer of polyisocyanate, polyether polyol and a surfactant, and the activator tank contains catalysts and water. Systems of this general type have become well-known over the years and are rather completely described in the readily available trade literature, complete with exemplification. See, for instance T. H. Ferrigno, Rigid Plastics Foams, Reinhold Publishing Corporation, New York 1963, entire text, and especially pages 53-72 and 118-123; and "Polyurethans", Encyclopedia of Polymer Science and Technology, Interscience Publishers New York 1969, especially volume 11, pages 537-548.
It is not for its thermal insulating properties alone that rigid plastic foam is used for void-filling of article shells. A very important desired function of the foam is, by adherence to the interior walls of the shell, unification and rigidification of the article as a whole, i.e. to give the article structural strength and a feel of integrity, though light in weight.
The prior art article and method have been subject to drawbacks which have gone unrelieved for a number of years. Particularly, it is time-consuming to hold a hollow molded shell in the mold or fixture while waiting for it to cool, cure or set sufficiently to be removed and remain free-standing without drooping, wilting, warping or otherwise becoming distorted, and then to fill the shell cavity with foaming plastic in a separate operation. The two step operation is unduly labor intensive, and may call for additional capital investment, e.g. for jigs or fixtures to hold the shells while they are being foam-filled, in order to prevent or minimize warpage and other distortion due to local over-filling with foam.
In addition, the prior art process is believed to consume more ingredients than theoretically necessary in manufacture of the foam filling, since an excess customarily is used in an attempt to insure that the entire void within the hollow shell is filled with foamed plastic. Even so, it is a troublesome artifact of use of the prior art method, especially on articles such as hospital trays which will be put through heat cycles, e.g. in the course of being washed in a dishwasher, that the shell balloons or buckles away from the foamed core fairly early in the life of the article. The article then has an unwanted feel; it no longer seems to be a structurally integral article, but one with a flexible skin over a definite internal body. Indeed the article may give-off a crackling, rattling, grating sound during use, as its shell contacts and comes away from the core. Needless to say, to the extent that the core is separated from the shell, the desirable structural strength is degraded.